Presently, Orthogonal Frequency Division Multiplexing (OFDM) is a transmission system widely used for various types of digital transmission, notably including terrestrial digital broadcasting and IEEE 802.11a. The OFDM method makes highly efficient use of frequencies by frequency-multiplexing a plurality of narrowband digitally-modulated signals using mutually-orthogonal sub-carriers.
Additionally, in the OFDM method, one symbol comprises a useful symbol as well as a guard interval, both of which are signals. As such, a portion of the useful symbol is replicated as the guard interval to produce intra-symbol periodicity. Thus, the influence of inter-symbol interference caused by multi-path interference is reduced, in that such a scheme offers superb resistance to such interference.
Analog television broadcasting is to cease around the world as worldwide frequency reconfiguration is being carried out. In Europe, demand is growing for HD (High Definition) broadcasting services in addition to the SD (Standard Definition) broadcasting services using DVB-T (Digital Video Broadcasting for Terrestrial) currently available. Thus the standardization of the second-generation European digital terrestrial television broadcasting system, DVB-T2, has advanced. The DVB-T2 broadcasting system is detailed in Non-Patent Literature 1.
As shown in FIG. 50, the DVB-T2 broadcasting system uses DVB-T2 frames, the structure of which includes a P1 symbol (P1 signal), one or more P2 symbols, and data symbols.
The P1 symbol is made to have an FFT (Fast Fourier Transform) size of 1 k. As shown in FIG. 51, guard intervals are arranged in front of and behind the useful symbol. In FIG. 51, the P1 symbol is shown in the time domain. Hereinafter, the guard interval arranged in front of the useful symbol interval is also termed the leading guard interval, while the guard interval arranged behind the useful symbol is also termed the trailing guard interval.
The guard interval of the P1 symbol differs from the guard interval used in the ISDB-T (Integrated Services Digital Broadcasting for Terrestrial) and DVB-T broadcasting systems thus far. In the P1 symbol, the guard interval arranged in front of the useful symbol (the leading guard interval) replicates the leading portion (Tc=59 μs) of the useful symbol (Ta=112 μs). Similarly, the guard interval arranged behind the useful symbol (the trailing guard interval) replicates the trailing portion (Tb=53 μs) of the useful symbol (Ta=112 μs). As shown in FIG. 51 and in Patent Literature 1, when these replicated portions are to be inserted, a predetermined frequency shift fSH (equivalent to the sub-carrier spacing of the P1 symbol) is first applied to the signal to be replicated before insertion. This process is expressed by the below-inscribed Math. 1.
                              p          ⁢                                          ⁢          1          ⁢                      (            t            )                          =                  {                                                                                          p                    ⁢                                                                                  ⁢                                          1                      A                                        ⁢                                          (                      t                      )                                        ⁢                                          e                                              j                        ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                                                      SH                            t                                                                                                                                ⁢                                                                                                                                                          0                    ≤                    t                    <                                          542                      ⁢                      T                                                        ⁢                                                                                                                                                                                  p                    ⁢                                                                                  ⁢                                          1                      A                                        ⁢                                          (                                              t                        -                                                  542                          ⁢                          T                                                                    )                                                        ⁢                                                                                                                                                                                542                      ⁢                      T                                        ≤                    t                    <                                          1566                      ⁢                      T                                                        ⁢                                                                                                                                                              p                  ⁢                                                                          ⁢                                      1                    A                                    ⁢                                      (                                          t                      -                                              1024                        ⁢                        T                                                              )                                    ⁢                                      e                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                      π                      ⁢                                                                                          ⁢                                              f                                                  SH                          t                                                                                                                                                                                      1566                    ⁢                    T                                    ≤                  t                  <                                      2048                    ⁢                    T                                                                                                                        0                  ⁢                                                                                                                                      otherwise                  ⁢                                                                                                                                                (                  Math          .                                          ⁢          1                )            
where p1(t) is the first P1 symbol, p1A(t) is the useful symbol, +fSH is the frequency shift, T is the time of one sample, post-IFFT, t is time, and the start time of the first P1 symbol is 0. In the DVB-T2 broadcasting system, for a bandwidth of 8 MHz, T= 7/64 μs and the time span of the useful symbol (hereinafter, useful symbol length) is 1024T=112 μs.
Also, as shown in FIG. 52, the P1 symbol as expressed in the frequency domain is seen to be composed of a plurality of Active carriers and a plurality of Null carriers (Unused carriers). Information is affixed to the Active carriers. For convenience, FIG. 52 illustrates the Null carriers with dashed arrows. In reality, the Null carriers carry no information and have no amplitude. As described in Patent Literature 2, the Active carrier positions are given by a predetermined sequence. That is, the positions are designated according to CSS (Complementary Sets of Sequences).
FIG. 53 shows the configuration of a typical P1 symbol demodulator 10001 demodulating the P1 symbol, as described by Non-Patent Literature 1. The P1 symbol demodulator 10001 includes a P1 position detector 10101, an FFT unit 10102, and a P1 decoder 10103.
The P1 position detector 10101 detects the position of the P1 symbol in the input signal (i.e., the P1 symbol demodulator 10001 input signal) and accordingly outputs P1 symbol position information to the FFT unit 10102. FIG. 54 shows the configuration of the P1 position detector 10101.
The P1 position detector 10101 includes a multiplier 10201, a delayer 10202, a complex conjugate calculator 10203, a multiplier 10204, an integral calculator 10205, a delayer 10206, a complex conjugate calculator 10207, a multiplier 10208, an integral calculator 10209, a delayer 10210, a multiplier 10211, and a peak detector 10212.
The P1 position detector 10101 input signal is input to the multiplier 10201. The multiplier 10201 multiplies the P1 position detector 10101 input signal by exp(−j2πfSHt) in order to apply a frequency shift that is the inverse of the +fSH frequency shift applied by the transmitter to the leading and trailing guard intervals of the first P1 symbol (applying a frequency shift of −fSH). The multiplier 10201 then outputs the result to the delayer 10202 and to the multiplier 10208. The delayer 10202 delays the multiplier 10201 output signal by Tc (=59 μs), a span equivalent to the leading guard interval time span (hereinafter, the length of the leading guard interval), and then outputs the signal so delayed to the complex conjugate calculator 10203. The complex conjugate calculator 10203 calculates the complex conjugate of the signal output by the delayer 10202 and outputs the resulting complex conjugate signal to the multiplier 10204. The multiplier 10204 calculates a correlation by multiplying the P1 position detector 10101 input signal and the complex conjugate calculator 10203 output signal, then outputs the correlated value so calculated to the integral calculator 10205. The integral calculator 10205 integrates the output signal from the multiplier 10204 over the length Tc of the leading guard interval, and then outputs the result to the delayer 10210. FIGS. 55A through 55C are schematic diagrams illustrating this signal processing. As shown in FIG. 55A, the leading guard interval obtained by frequency shifting the P1 position detector 10101 input signal by −fSH and then delaying the result by the length Tc of the leading guard interval (shown in the lower portion of FIG. 55A) is identical to the leading part of the useful symbol within the P1 position detector 10101 (shown in the upper portion of FIG. 55A). A correlation appears in this portion, as shown in FIG. 55B. Given that other parts of the signals are not identical, no correlation appears therein. The peak shown in FIG. 55C is the effect of integrating the correlated value shown in FIG. 55B over the length Tc of the trailing guard interval.
Meanwhile, the P1 position detector 10101 input signal is input to the delayer 10206. The delayer 10206 delays the P1 position detector 10101 input signal by Tb (=53 μs), a span equivalent to the trailing guard interval time span (hereinafter, the length of the trailing guard interval), and then outputs the result to the complex conjugate calculator 10207. The complex conjugate calculator 10207 calculates the complex conjugate of the signal output by the delayer 10206 and outputs the resulting complex conjugate signal to the multiplier 10208. The signal input to the multiplier 10208 is the result of the multiplier 10201 multiplying the P1 position detector 10101 input signal by exp(−j2πfSHt). The multiplier 10208 calculates a correlation by multiplying the multiplier 10201 output signal (the P1 position detector 10101 input signal with a frequency shift of −fSH applied thereto) and the complex conjugate calculator 10207 output signal, then outputs the correlated value so calculated to the integral calculator 10209. The integral calculator 10209 integrates the multiplier 10208 output signal over the length Tb of the trailing guard interval, and then outputs the result to the multiplier 10211. FIGS. 56A through 56C are schematic diagrams illustrating this signal processing. As shown in FIG. 56A, the trailing guard interval obtained by frequency shifting the P1 position detector 10101 input signal by −fSH (shown in the upper portion of FIG. 56A) is identical to the useful symbol within the P1 position detector 10101 with the trailing part delayed by the length Tb of the trailing guard interval (shown in the lower portion of FIG. 56A). The correlation appears in this part, as shown in FIG. 56B. Given that other parts of the signals are not identical, no correlation appears therein. The peak shown in FIG. 56C is the effect of integrating the correlated value shown in FIG. 56B over the length Tb of the trailing guard interval.
The signal output from the integral calculator 10205 is input to the delayer 10210. The delayer 10210 delays the signal output from the integral calculator 10205 to the match in the signal output from the integral calculator 10209 for output to the multiplier 10211. The multiplier 10211 multiplies the signal output from the integral calculator 10209 by the signal output from the delayer 10210, and then outputs the product to the peak detector 10212. Thus, the peaks are made more prominent by matching the peaks in the correlated value integral taken for the leading guard interval to the peaks in the correlated value integral taken for the trailing guard interval. The peak detector 10212 detects the position of the P1 symbol within the P1 position detector 10101 input signal (i.e., the signal input to the P1 symbol demodulator 10001) by detecting the peak position in the signal output from the multiplier 10211. The peak detector 10212 accordingly outputs position information for the P1 symbol to the FFT unit 10102 shown in FIG. 53. Given the presence of a delayed wave, a peak correlation appears in correspondence to the level and position of the delay.
The FFT unit 10102 shown in FIG. 53 performs a FFT (Fast Fourier Transform) on the signal input from the P1 symbol demodulator 10001 (a time-domain signal) in accordance with the P1 symbol position information, thus obtaining a converted frequency-domain signal for output to the P1 decoder 10103. The P1 decoder 10103 executes a decoding process on the P1 symbol using the Active carriers in the frequency-domain signal, calculates the values of the S1 and S2 signals added to the P1 symbol to discern information therefrom, such as the FFT size and MISO/SISO information.
Incidentally, the DVB-T2 broadcasting system includes FEF (Future Extension Frames) so that future broadcasting systems can broadcast using time multiplexing. Accordingly, broadcasting systems other than DVB-T2 are made possible. FIG. 57 shows the positional relationship between FEF and DVB-T2 frames. The head of an FEF part is a P1 symbol, much like that of a DVB-T2 frame. However, the information affixed to the P1 symbol is different from that used in the DVB-T2 broadcasting system. Therefore, a receiver implementing the DVB-T2 broadcasting system (hereinafter, a DVB-T2 receiver) demodulates the P1 symbol of the FEF part with the P1 symbol demodulator 10001, and can then acknowledge the presence of an FEF part by using the information affixed to the symbol.